chore(travis): revert blacklist Dart SDK 1.7.0-dev.2.0
chore(travis): revert blacklist Dart SDK 1.7.0-dev.2.0

This reverts commit e2350cac61dc33053504c70be4d1d5411e0f2040.

Current Dart dev sdk version has advanced past the buggy version.

Closes #1447

ronlobo/angular.dart,SibuStephen/angular.dart,rkirov/angular.dart,SibuStephen/angular.dart,hitesh97/angular.dart,rkirov/angular.dart,winstonewert/angular.dart,ronlobo/angular.dart,ronlobo/angular.dart,rkirov/angular.dart,trinarytree/angular.dart,trinarytree/angular.dart,hitesh97/angular.dart,ronlobo/angular.dart,rkirov/angular.dart,winstonewert/angular.dart,trinarytree/angular.dart,winstonewert/angular.dart,SibuStephen/angular.dart,SibuStephen/angular.dart,winstonewert/angular.dart,trinarytree/angular.dart,hitesh97/angular.dart,hitesh97/angular.dart